Just ordered a 18" upper and the stock I have is a 6 position collapsible with a carbine buffer and spring. Do I have to get a different stock? Doesn't it make a difference on a mid length? What would happen if the incorrect combination was used? Sorry for all the questions!

The buffer and spring is dependant on the stock type not the upper type. What you have will work fine.

People changing buffer weights and springs are trying to fine tune or fix a problem with their rifle.  Like mentioned, spring and buffer are dependent on the stock because of the length of travel for the buffer to absorb recoil and cycle the weapon, get the buffer and spring for the type of stock you have, it should work with any barrel, gas system combination.

Correct me if I'm wrong, but I believe the longer Rifle stock buffer system should supply a softer recoil, while the shorter carbine collapsible stock buffer system should be a little more harsh.  The gas system should effect the recoil more than the buffer system.  If you have a mid-lenth gas system, a normal collapsible stock buffer system should work just fine and have a very reasonable recoil.

That makes sense. My thinking was a little backwards, but that clears it up. Thank you

You can try changing to a heavier buffer (H1, H2, etc) if the recoil is too harsh. Also check out the Spike's SP-T2 heavy buffer.

Here is a crazy question! My friend asked me what would happen if he used a carbine buffer in a rifle or vise versa.... I did not have an answer. What would happen, if anything?

Carbine buffer can be used in a rifle tube if you use a spacer.  Rifle buffer will not work in a carbine tube, too long.

What kind of spacer? Not sure I understand. Then after the spacer you use the carbine spring with the carbine buffer in the rifle stock tube?

Correct.

RRA makes a spacer for their 9mm conversion that works perfectly for using a carbine spring and buffer in a rifle tube, costs about $8.  I just went this route when switching from a collapsable stock to a fixed stock.  Figure I already had the carbine spring and buffer which worked perfectly with my rifle, so why not keep em.

Ya thats pretty much the same problem we had.... Thanks for the info! It will help a ton!